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#pragma once
#include <random>

namespace Falcor
{
    /** Implements the alias method for sampling from a discrete probability distribution.
    */
    class FALCOR_API AliasTable
    {
    public:
        using SharedPtr = std::shared_ptr<AliasTable>;

        /** Create an alias table.
            The weights don't need to be normalized to sum up to 1.
            \param[in] weights The weights we'd like to sample each entry proportional to.
            \param[in] rng The random number generator to use when creating the table.
            \returns The alias table.
        */
        static SharedPtr create(std::vector<float> weights, std::mt19937& rng);

        /** Bind the alias table data to a given shader var.
            \param[in] var The shader variable to set the data into.
        */
        void setShaderData(const ShaderVar& var) const;

        /** Get the number of weights in the table.
        */
        uint32_t getCount() const { return mCount; }

        /** Get the total sum of all weights in the table.
        */
        double getWeightSum() const { return mWeightSum; }

    private:
        AliasTable(std::vector<float> weights, std::mt19937& rng);

        // Item structure for the mpItems buffer.
        struct Item
        {
            float threshold;                ///< If rand() < threshold, pick indexB (else pick indexA)
            uint32_t indexA;                ///< The "redirect" index, if uniform sampling would overweight indexB.
            uint32_t indexB;                ///< The original / permutation index, sampled uniformly in [0...mCount-1]
            uint32_t _pad;
        };

        uint32_t mCount;                    ///< Number of items in the alias table.
        double mWeightSum;                  ///< Total weight of all elements used to create the alias table.
        Buffer::SharedPtr mpItems;          ///< Buffer containing table items.
        Buffer::SharedPtr mpWeights;        ///< Buffer containing item weights.
    };
}
